1. Field of the Invention
The present invention relates to a tilt cylinder unit for locking an outboard engine on a boat hull or unlocking the outboard engine from the boat hull under strong forces applied thereto.
2. Description of the Prior Art
Some small power-driven boats such as motor boats have an outboard engine vertically angularly movably supported on a boat hull and a tilt cylinder unit operatively coupled between brackets joined to the outboard engine and the boat hull, the tilt cylinder unit having a tilt lock function. Such an outboard engine assembly is disclosed in U.S. Pat. No. 4,925,411 and Japanese patent publication No. 2-58155.
As shown in FIG. 1 of the accompanying drawings, the tilt cylinder unit disclosed in U.S. Pat. No. 4,925,411 has a piston 101 fixed to an end of a rod 100 and slidably fitted in a cylinder 105. The piston 101 divides the interior space of the cylinder 105 into a first chamber S1 which is expanded when the tilt cylinder unit is extended and a second chamber S2 which is compressed when the tilt cylinder unit is contracted. The rod 100 extends through the second chamber S2. These chambers S1, S2 are interconnected by a communication passage 103 having an on-off valve 102. The piston 101 has a check valve 104 which interconnects the chambers S1, S2. When a large force is applied to the tilt cylinder unit, the check valve 104 is opened to allow working oil to flow from the second chamber S2 into the first chamber S1. A. gas G is filled in the second chamber S2 to compensate for a change in the volume of the second chamber S2 as the rod 100 moves into and out of the second chamber S2. When the outboard engine is operated to propel the motorboat forwardly with the on-off valve 102 closed, since the working oil in the first chamber S1 is completely locked, the tilt cylinder unit is not contracted, and the outboard engine is kept at a certain constant angle with respect to the boat hull. However, when the outboard engine is operated to reverse the motorboat with the on-off valve 102 closed, since the outboard engine tends to move backwards in an upward direction, the tilt cylinder unit is subjected to a force tending to extend the tilt cylinder unit. At this time, the gas G filled in the second chamber S2 is compressed, permitting the outboard engine to be displaced for an interval corresponding to the amount by which the gas G is compressed.
The tilt cylinder unit disclosed in Japanese patent publication No. 2-58155 is shown in FIG. 2 of the accompanying drawings. The disclosed tilt cylinder unit differs from the tilt cylinder unit shown in FIG. 1 in that a gas G is filled in the first chamber S1. When the outboard engine is operated to reverse the motorboat forwardly with the on-off valve 102 closed, since the working oil in the second chamber S2 is completely locked, the tilt cylinder unit is not contracted, keeping the outboard engine at a certain constant angle with respect to the boat hull. However, when the outboard engine is operated to propel the motorboat forwardly, especially in shallows, the gas G in the first chamber S1 is compressed and exerts a resilient reactive force against forces tending to contract the tilt cylinder unit. At this time, the outboard engine is caused to bounce and becomes positionally unstable.